1. Field of the Invention
This invention concerns a process for producing silicon carbide whiskers, and more specifically, it relates to a process for producing silicon carbide whiskers which are linear, of high aspect and high purity at a high yield.
2. Description of the Prior Art
Silicon carbide whiskers have been used in various application uses, and for example, in the case of using them as various reinforcing materials by utilizing their strength, it is required that the fiber length is long and the aspect ratio defined as a ratio of the length relative to the cross sectional diameter of the fiber is high.
As a typical process for producing silicon carbide whiskers, there has been known, for example, a method of uniformly mixing powdery silicon-containing starting material and carbon-containing starting material and reacting them under heating at a high temperature as described, for example, in Japanese Patent Application Laying Open No. Sho 58-14570. Further, for intensely mixing the starting material as described above, it has also been proposed a method of mixing to mold a powdery silicon-containing starting material and a carbon-containing starting material and then heating them, for example, in Japanese Patent Application Laying Open No. Sho-58-20799.
However, in such prior art methods, it has been difficult to obtain silicon carbide whiskers of long fiber length and high aspect ratio, and in addition, the resultant silicon carbide whiskers are generally of low purity containing unreacted silicon-containing starting material such as silica or the like and purifying treatment such as using fluoric acid or the like is necessary before use.
Further, it is not easy in the former method to separate the thus formed silicon carbide whiskers from the starting material. In addition, in the case of preparing silicon carbide whiskers through the heating reaction of a solid-like silicon-containing starting material and a carbon-containing starting material, since impurities contained in the starting materials are generally dispersed uniformly and contained in the resultant whiskers, if such impurities are contained in a great amount, they have to be separated and removed. While on the other hand, if it is intended to save the separating and removing step for the impurity, it is necessary to limit the impurity content in the starting material to less than a certain level.
In order to overcome the foregoing problems, the present inventors have already proposed a process of bringing the silicon-containing material and the carbon-containing material into contact and reaction at a temperature higher than 1,000.degree. C. wherein a metal chloride typically represented by sodium chloride to be present together (refer to Japanese Patent Publication No. 58-28758), or a process of mixing and introducing hydrogen chloride or chlorine to a mixture of a finely powderous silicon starting material, a carbon starting material and a catalyst under a non-oxidizing atmosphere and bringing into reaction at a temperature from 1,350.degree. C. to 1,600.degree. C. (refer to Japanese Patent Application Laying Open No. Sho 58-251964), but they have not been quite satisfactory in view of the yield, the aspect ratio, etc.